Food Science and Technology Department

 

Date of this Version

4-2011

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Agricultural and Biological Systems Engineering, Under the Supervision of Professor Jeyamkondan Subbiah. Lincoln, Nebraska: May, 2011

Copyright 2011 Krishnamoorthy Pitchai

Abstract

Microwave (MW) ovens are used extensively for heating a variety of not-ready-to-eat food products. It is vital to achieve target temperature uniformly throughout the food to inactivate foodborne pathogens to assure safety. Non-uniform heating of foods in microwave ovens is the major concern in assuring microbiological safety of such products. The non-uniform heating of foods in domestic microwave ovens is due to complex interactions of microwaves with foods. A comprehensive coupled electromagnetic and heat transfer model was developed using finite-difference time-domain based numerical method to understand the complex interaction of microwaves with foods. Simulation parameters such as cell size, heating time step, number of iterations for steady state electromagnetic field were optimized. The coupled model was validated by heating a cylindrical model food (1% gellan gel) for 30 s in a microwave oven (700 W). The model was validated qualitatively by measuring the product temperature profiles on three planes in the gel and compared to the thermal images. Quantitative validation was performed by measuring the temperature of the gel at 12 locations using fiber optic sensors. Model spatial temperature profiles agreed well with the thermal image profiles at 2.45 GHz frequency. The root mean square error values ranged from 0.53 to 4.52°C, with an average value of 2.02°C.

Adviser: Jeyamkondan Subbiah

Microwave (MW) heating is fast and convenient, but is highly non-uniform. When a food product contains raw or partially cooked food components, non-uniform heating can result in inadequate cooking, leading to a microbiologically unsafe product. A mathematical model of microwave heating helps to understand non-uniform temperature distribution in domestic microwave ovens and a useful tool for food product developers and microwave oven manufacturers. The first objective of this study was to develop a mathematical model to predict spatial and temporal variations in temperature of a model food during microwave heating.

A mathematical model was developed by solving coupled Maxwell‘s electromagnetic and Fourier's heat transfer equations using finite-difference time-domain (FDTD) method in QuickWave v7.5 software. The model was used to describe the heating of a gellan gel cylinder for 30 s in a 700 W domestic microwave oven. The model domain included the magnetron, typical waveguide, cavity and turntable. Optimization of modeling parameters such as computational meshing size, heating time step, frequency, and electric field strength was performed to increase accuracy of the prediction of the temperature profile. The model was validated by conducting microwave heating experiments to observe time-temperature and spatial-temperature profiles using fiber optic thermocouples and thermal imaging camera, respectively. A good qualitative agreement between simulation and experimental temperature profiles was observed. Results of quantitative analysis of point measurement of time-temperature profile showed that average root-mean squared error of the 12 locations was 2.02°C.

Rate of microwave heating depends on dielectric and thermo-physical properties of food products. However, non-uniform heating is highly contributed by inherent standing wave patterns of electromagnetic waves in a MW cavity. Non-uniform heating has become an important factor in assessing the safety of microwave-heated products. The second objective of this study was to quantify non-uniform heating in a range of domestic microwave ovens. A custom designed 12" diameter polypropylene container with 36 equal volume compartments was fabricated to assess non-uniform heating in 19 different domestic microwave ovens. A container filled with 1 liter of water was subjected to 2 min heating in microwave ovens. Immediately after MW heating, a hedgehog of 30 T-type thermocouples was inserted in the compartments to record the final temperature. Effect of radial distance and sectors on temperature variation and heating rate within a cavity was studied to recommend the best location for placing food on a turntable. The results suggested that the best place to put food in a microwave oven is at the edge of the turntable to achieve faster heating rate and better heating uniformity.